linux-kernel - Re: [PATCH] x86: Configure NX support earlier in setup

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Date:	Mon, 14 Jul 2014 19:22:25 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Stuart Hayes <stuart.w.hayes@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de,
	mingo@...hat.com, x86@...nel.org, linux-kernel@...r.kernel.org,
	matt.fleming@...el.com, bp@...e.de
Subject: Re: [PATCH] x86: Configure NX support earlier in setup_arch

On Wed, Jul 09, 2014 at 07:56:29PM -0500, Stuart Hayes wrote:
> Well... I got this issue because a co-worker tripped over it. He had
> NX disabled in BIOS for some reason, and found that linux wouldn't
> boot--it hung right after grub2. I guess it took a while to figure out
> that it was the fact that NX was disabled that caused linux not to
> come up--and that could happen to other people. I don't know of any
> real-world scenarios in which someone would actually prefer to run a
> recent linux kernel with NX disabled, though.
>
> It looks like some of the other boot paths into the kernel
> automatically clear the XD_DISABLE bit in the MISC_ENABLE MSR in the
> CPU (in verify_cpu), but that doesn't happen when grub2 jumps to
> startup_64 in arch/x86/boot/compressed/head_64.S. I guess instead
> of this patch, I could try to make a patch that turns NX back on
> (somewhere in startup_64), but since the kernel already supports NX
> being disabled, so I thought maybe just fixing that would be better. I
> didn't like seeing the kernel just die without giving any indication
> of what the problem is.

Well, hpa and I were talking about this briefly and this NX disabling
in the BIOS is probably for some broken legacy applications/OSes. Linux
enables NX unconditionally very early because disabling it is a very bad
idea anyway, security-wise.

So, if this is just a random trip over of a co-worker and doesn't have
any sensible use case, I'd rather leave it as is an don't fix it at all.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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